Due to the way the game is coded you can't just simply add a bunch of new options. Every customization slot added would cause exponentially more lag in player populated areas. We're basically at the reasonable limit already which is why you can't even use a fashion accessory along with a minion or mount. They all have to share the same slot to save space. Any new option added would have to be replacing something currently existing.

Body sliders are never happening. All gear has to be manually adjusted to body shapes. Adding an entirely new body type would be close to the same amount of effort as adding a new race. I'm fairly sure tattoos/body scars wouldn't work either because skin textures are baked into the gear and would overwrite it.

Frankly I think the customization in this game is already good enough. They just need to expand on what's already there. There's something to be said for how difficult it is to create a truly ugly character in this game. I've seen what people will choose to create when given infinite freedom and it isn't good.